There are various devices and methods used to assess the direction of fixation of an eye. One example of a fixation measurement device is described in U.S. Pat. No. 6,027,216 (“the '216 patent”), the contents of which are hereby incorporated by reference in their entirety.
In general, the device in the '216 patent assesses the direction of fixation by: (1) directing incident light to illuminate the fundus of the eye; (2) detecting light reflected from the fundus; (3) determining polarization-related changes between the incident light and the reflected light; and (4) correlating the polarization-related changes with changes known to occur with known fixation states of the eye.
The '216 patent discloses devices which utilize a continuous scan of retinal areas to assess the direction of fixation of the eye. One example of such a device is shown in FIG. 1. The device includes a light source 101, a beam splitter 102, a polarization beam splitter 103, photodetectors 104A and 104B, a motor 105 having a rotatable shaft, a first concave mirror 106, and a second concave mirror 107.
The light source 101 provides a diverging beam of polarized light which passes through beam splitter 102 and is incident on the first concave mirror 106. The first concave mirror 106 is mounted in a tilted fashion on the shaft of the motor 105 such that the first concave mirror wobbles 106 slightly when the shaft rotates. The first concave mirror 106 forms an image of the light source 101 on the surface of the second concave mirror 107. The second concave mirror 107 is stationary and is larger than the first concave mirror 106. As the shaft of motor 105 rotates, the image of the light source 101 on the surface of second concave mirror 107 is continuously scanned about a circular path. The curvature of stationary second concave mirror 107 can be chosen such that an image reflected from the spinning first concave mirror 106 is formed directly at the eye 108. All the light leaving the spinning first concave mirror 106 is imaged by stationary second concave mirror 107 to pass through a stationary exit pupil of the device, designated by the dashed circle, which overfills the pupil of the eye 108. The eye 108 sees the spinning image of the light source 101 in the form of a circle of light on the surface of stationary second concave mirror 107. A continuous annular scan of retinal areas is thus achieved by the light incident on the eye 108.
In order to allow for rapid measurements of the light reflected from the fundus, it is desirable to operate the above-described scanning at a scanning rate of at least 100 Hz and preferably at rates of 200 Hz or more. Scan rates at 200 Hz or more permit measurements to be obtained when working with subjects that may be less than fully cooperative, as is commonly the case with very young children. Such rates require the mechanical rotation of the first concave mirror 106 at rates which place special requirements on the mounting of the first concave mirror 106. In the case of retinal birefringent scanning, the first concave mirror 106 is tilted at an angle of approximately 1.5 degrees (to generate a tilt of approximately 3 degrees), and the first concave mirror 106 is then rotated about the axis of the chief ray of the optical beam.
Unfortunately, the tilt of the first concave mirror 106 can create a problem when it is rotated at high rates. Although the first concave mirror 106 is mechanically balanced when not rotating, the introduction of spin generates forces on the first concave mirror 106 (and the mechanical apparatus holding the mirror) that are not balanced.
For a flat disk, normal spin performed on the flat disk would have forces acting on the mass, but these forces all point away from the center, and have a vector that is normal to the axis of spin. For a mechanically balanced mass (with the center of gravity located precisely on the axis of rotation), the sum of all the force vectors for all divisible portions of the rotating mass cancel, and there is no net force vector.
However, this is not the case for a tilted mass being rotated about the center of gravity, such as the first concave mirror 106 shown in FIG. 1. FIG. 2 illustrates the torque exerted on the first concave mirror 106. The vertical dotted line separates the upper and lower mass portions of the first concave mirror 106. The dot aligned with the rotation axis indicates the center of gravity of the entire concave mirror 106, the dot above the aligned dot indicates the center of gravity for the upper mass portion of the concave mirror 106 and the dot below the aligned dot indicates the center of gravity for the lower mass portion of the concave mirror 106.
As the tilted mass portions are rotated about the center of gravity, the top half of the mass will have a force vector F1 outward and above the center of gravity, whereas the bottom half of the mass will have a force vector F2 that is outward and below the center of gravity. The result is a speed dependent torque that is exerted onto the concave mirror 106 as the two forces act against each other. Not only is the torque speed dependent (torque increases as speed increases), but it is also continuously oriented parallel to the axis of tilt of the concave mirror 106. Therefore, the torque has a similar mechanical vibration as if there was an off-axis mass.
If the concave mirror 106 is held rigid using a mechanical method, then the torque exerted will perform work and rotate the entire mechanical assembly, if even a small amount. For low speeds, this torque is small and the magnitude of the movement of the entire device which includes the concave mirror 106 is small. But at higher speeds, the torque can become excessively large, and the entire device can vibrate excessively. Such vibration can place undesirable stress on some of the components of the device, possibly leading to fatigue in the components and eventually failure.
One known approach to minimize excessive vibration with a rotating tilted disk is to use a symmetrical disk which is of the same mass, size and shape of the tilted disk, but angled opposite to the angle of the tilted disk. FIG. 3 illustrates a pair of rotating, tilted disks 301 and 302 which have a symmetric mass and tilt about an imaginary centerline lying between them. The figure would be similar for a pair of rotating, tilted concave mirrors such as those shown in FIG. 1. For the approach shown in FIG. 3, symmetrical disk 302 rigidly attached to disk 301. The assembly of FIG. 3 is constructed such that the overall mass is balanced when there is no rotation. Additionally, due to the symmetrical arrangement of the disks 301 and 302, the torque exerted by the two masses during rotation also cancels out.
There are still potential shortcomings with this approach. Most notable is that the mass of the rotating object has doubled. For a device that performs scanning, this places extra time delay between the time when the motor is started and the time when the needed rotational speed has been achieved. This can make the device unsuitable for stopping and starting, and may require that the device is simply left with the motor spinning so that it is ready to use. Another potential shortcoming with this approach is that the tilted disk may have a shape that is not a simple flat disk but rather a concave disk such as the first concave mirror of the '216 patent. In this situation, a symmetrical concave mirror could be tilted at precisely the same angle (but in an opposite direction) as the first concave mirror. However, the additional component and the additional steps needed to fabricate this arrangement would result in a higher cost for the device. Additionally, there is a lack of machinery which is optimized for fabricating such assemblies and therefore the symmetrical disk approach can involve extra time in manufacturing in addition to the extra materials.
Another potential shortcoming with the symmetrical disk approach is that it can also be complex to resolve or correct for residual errors in manufacturing, which are virtually unavoidable for such an arrangement. Such errors generate vibrations, which need to be corrected. These types of errors are inherently difficult to correct because the assembly needs to be stopped in order to be adjusted, but the motor must be spinning in order to observe the vibration. Furthermore, making the necessary adjustments can be very time consuming.